1. Field of the Invention
The present invention relates to the field of amplifiers, buffers, power supply circuits and the like wherein overload protection is needed and wherein operation capabilities near the power supply rail are required.
2. Prior Art
This invention is particularly relevant in the field of amplifiers, buffers, power supply circuits, etc. where the output load may be ill defined for many applications. For example, power supply circuits may be subject to short circuit loads in fault conditions and/or have loads that may be highly capacitive so that at startup, unless constraints are imposed, extremely high values of initial current would result. In general most electronic functions that supply large amounts of power (in an absolute sense) into loads have to have protection against overloads. Fuses, circuit breakers and current limiting schemes are commonly used for power source protection.
This disclosure describes a method of protecting an error amplifier, buffer or equivalent using, but not limited to, a MOS pass transistor amongst other circuitry, connecting a power source to a load.
To illustrate the problems that the present invention is trying to solve, the low dropout voltage positive series regulator of FIG. 1 will be described. This regulator employs a conventional P-MOS pass element and operates with an input supply voltage greater than its output voltage. The load at the output will generally consist of a resistive element R.sub.L (dissipative) and a parallel capacitive element C.sub.L (generally used for stability of the regulator's error amplifier and to compensate for the normally poor high frequency power supply and load regulation of the same). It is also one of the requirements of the regulator that it can survive an output short circuit condition.
FIG. 1 shows a simple block schematic of the low dropout voltage regulator. Low dropout voltage refers to the low voltage that would occur across the P-MOS transistor as the supply voltage is reduced to a value slightly greater than the output voltage. Stated alternatively, low dropout voltage regulators that can provide a voltage regulated power output up to voltages approaching the input supply voltage to the regulator. Many regulators have dropout voltages of greater than 1 volt. The regulator shown has the capability of providing dropout voltages of less than 0.1 volt at moderate to low output currents.
One of the problems of low dropout voltage regulators is how to determine what the current is through the pass element without affecting the dropout voltage. One method is to place a resistor in series with the pass element and to monitor the voltage across this resistor. For low dropout voltage regulators, this is undesirable because the voltage produced across the resistor adds to the total dropout voltage.
As an example of a practical requirement for a low dropout voltage regulator, a power supply will be considered having an output current requirement of 250 milliamperes at 5 volts with a dropout voltage of 125 mv when the input supply is 5 volts. This implies an "ON" or "Saturation" resistance of 0.5 ohm for the pass element in dropout. The pass transistor would therefore have to be designed such that, with a gate to source voltage of not more (and probably less) than the input supply voltage, the transistor would have an ON resistance of 0.5 ohm. The problem is that with such a transistor, if the output is shorted to GND, the transistor will be able to supply several amps of current depending upon the parameters of the transistor (threshold voltage, channel carrier mobility, etc.). This is clearly undesirable from both an excessive power supply consumption consideration because of the limited ability to dissipate that power in the pass transistor without exceeding potentially dangerously temperatures, and the limited ability of the connections to the pass transistor to handle high values of current. Since power is the product of voltage and current, either voltage or current can be high but not both simultaneously. In many applications, this voltage can have any value up to the input power supply voltage. Therefore, the maximum value(s) of current in the pass element must be controlled to protect the system. The multilevel control of the current through the pass element is what the invention achieves.
It is almost impossible to define an ideal current limiting scheme for a general purpose amplifier, regulator, etc. since the desired operating, overload and transient conditions can be so varied. For example, for a DC system where startup time is unimportant, and with an output requirement of say 100 ma, a scheme that limits the maximum output current to 110 ma may be fine if there is not a dropout requirement. This is because in order to obtain the lowest possible dropout voltage, it is desirable, when using a MOS pass transistor, to apply as high a gate to source voltage as possible. Accordingly, the on resistance of this transistor is very low and if a current detection scheme is not used, as the dropout voltage is increased, the pass transistor current could increase to many times the desired maximum value. If as, in most real case situations, the load has AC requirements such as rejection of high frequency noise, and high frequency error amplifier loop stability, etc., the load has to be decoupled with a large capacitor which places different and usually much more severe restrictions on protection schemes in addition, of course, to the dropout requirements. This is the reason for there being so many current limiting and power protection circuits in current usage.